An angular velocity sensor, which is one of conventional inertial force sensors, is described below. A conventional angular velocity sensor includes a detecting element that typically has a tuning-fork shape, H-shape, T shape, or disk shape. An angular velocity is detected by vibrating this detecting element and electrically detecting distortion (strain) of the detecting element caused by occurrence of Coriolis force. For example, if a vehicle is disposed on an X-Y plane defined of X-axis and Y-axis assuming that the X-axis, the Y-axis, and Z-axis are substantially perpendicular to each other, the angular velocity sensor in the navigation system detects the angular velocity around the Z-axis of the vehicle.
FIG. 11 is a perspective view of a detecting element of a conventional angular velocity sensor. FIG. 12 is a sectional view taken along line 12-12 in FIG. 11. Detecting element 51 has a tuning-fork shape, and includes two arms 52 (52A and 52B) and base 53 connecting arms 52.
Drive electrode 54 for driving and vibrating arm 52 and sensor electrode 55 for sensing distortion of arm 52 caused by angular velocity are disposed on each arm 52. For example, each of drive electrode 54 and sensor electrode 55 is formed of upper electrode 57 and lower electrode 58 with piezoelectric body 56 in between.
Detecting element 51 is disposed, for example, standing along the Z-axis direction with respect to the X-Y plane. In this state, arms 52 are driven and vibrated along the X-axis direction to detect distortion caused by the angular velocity around the Z-axis by arms 52. The angular velocity around the Z-axis is detected by this detection. A driving and vibration face (X-Z plane) of each arm 52 and a distortion face (Y-Z plane) of each arm 52 are perpendicular to each other, and distortion directions are opposite between arm 52A and arm 52B. For example, if arm 52A is distorted in a positive direction of Y-axis, arm 52B is distorted in a negative direction of Y-axis. This type of angular-velocity sensor is disclosed in PTL 1.
In general, sensitivity of sensor electrode 55 can be improved by reducing a frequency difference (mistuned frequency) between a frequency for excitation and a frequency for sensing on detecting the angular velocity. This also results in improving the detection sensitivity of angular velocity. In addition, the frequency for excitation and the frequency for sensing are also designed to be resonance frequencies of detecting element 51, respectively. However, a sufficient gain may not be obtained if detecting element 51 is designed based only on these indexes.